1. Field of the Invention
The present invention relates to a phase ambiguity elimination device and, more particularly, to a phase ambiguity elimination device which uses a detected synchronizing word in eliminating phase ambiguity from a QPSK signal. The synchronizing word may be a unique word (UW) of frame synchronizing system used in a modulation/demodulation apparatus of a satellite communication, quadrature phase shift keying (QPSK) system.
2. Description of the Related Art
In a frame synchronizing system of a QPSK modulation system, each of the Pch and Qch data signals is provided with a UW. The UW is detected and used for phase synchronizing purposes at a receiving side. However, there are four phase states (P, Q), (Q, P), (P, Q) and (Q, P) due to phase slip, as described in, for example, Japanese Patent Laid-open Publication No. 134951/1984 (JP-A-59-134951). P and Q stand for inversions of P and Q channel signals, respectively. Therefore, when such data is demodulated in the receiving side, there is a phase ambiguity. In order to remove the ambiguity, it is necessary to determine the phase of the received signals, and correct the phase, when necessary, to a determined, correct phase. This may be achieved by using a phase ambiguity elimination or removing device, as it may also be called. The phase ambiguity removing device compares a demodulated UW with a reference UW stored in the receiving side, calculates an error value and decides the correct phase according to the error value.
As described above, since the position of the UW in a frame is predetermined in accordance with system criteria, subsequent UWs can be detected at fixed intervals once a first UW is detected.
However, if there is a failure to detect the UW then a change in phase is not detected and the last detected phase information is used continuously, since there is no new phase information. As a result, if the phase of the received data changes due to phase slip, the phase ambiguity removing device can not properly correct the phase for it operates on received data with erroneous phase information. Consequently, the received data becomes different from the transmitted data, resulting in an error.